The invention relates to a method for controlling grain spacing of a columnar superalloy casting.
A superalloy includes nickel, cobalt, nickel-iron or iron-based heat resistant alloys that have superior strength and oxidation resistance at high temperatures. The superalloy can contain chromium to impart surface stability and one or more minor constituents, such as molybdenum, tungsten, columbium, titanium or aluminum, for strengthening purposes. The physical properties of a superalloy make it particularly useful for the manufacture of a gas turbine component.
A grain is an individual crystal in a polycrystalline solid. A grain boundary is an interface between individual crystals. The crystal grain characteristics of a superalloy can determine superalloy properties. For example, the strength of a superalloy is determined in part by grain spacing. At low temperatures, grain boundaries impede dislocation motion. Hence, fine grain equal axial structures are preferred for low temperature applications. At high temperatures, deformation processes are diffusion controlled. Diffusion along grain boundaries is much higher than within the grains. Hence, large-grain spacing structures can be stronger than fine grain structures in high temperature applications. Generally, failure originates at grain boundaries oriented perpendicular to the direction of an induced stress. By casting a superalloy to produce an elongated columnar structure with unidirectional crystals aligned substantially parallel to the long axis of the casting, the number of grain boundaries normal to the primary stress axis can be minimized.
Directional solidification is a method that is used for producing turbine blades and the like with columnar crystalline structures. Generally, a crystalline growth structure is created at the base of a vertically disposed mold defining a part and a solidification front is propagated through the structure under the influence of a moving thermal gradient. During directional solidification, crystals of nickel, cobalt or iron-based superalloys are characterized by a xe2x80x9cdendriticxe2x80x9d morphology. Dendritic refers to a form of crystal growth where forming solid extends into still molten liquid as an array of fine-branched needles. Spacing between the needles in the solidification direction is called xe2x80x9cprimary dendrite arm spacing.xe2x80x9d Spacing of side branches or arms along a needle""s length is termed xe2x80x9csecondary dendrite arm spacing.xe2x80x9d Both primary and secondary dendrite arm spacing are functions of cooling rate. Cooling rate is the product of solidification rate and thermal gradient at a solid liquid interface.
Solidification rate kinetics vary with crystallographic orientation. For a fixed driving force, the solidification rate in nickel based superalloys is typically highest in the crystallographic unit cell edge direction ( less than 100 greater than  direction).
One desired macrostructure of a directionally solidified superalloy consists of grains elongated along the direction of solidification so that grain boundaries are aligned in the solidification direction. It is further desired that the crystallographic unit cell edge direction  less than 100 greater than  of the grains be parallel to the solidification direction to provide improved mechanical properties. If two grains are growing side-by-side into a temperature gradient, the grain with growth axis closest to a  less than 100 greater than  direction grows faster. The faster growing grain also spreads laterally. Lateral spread of a grain occurs by growth of secondary arms. If two grains are growing side by side into a liquid, and one grain leads the other by secondary arm spacing, the leading grain will extend a secondary arm in front to pinch off the lagging grain. This phenomenon is termed xe2x80x9ccompetitive growth.xe2x80x9d Until competitive growth achieves a structure of only grains close to the  less than 100 greater than  direction, the grain boundaries will not be parallel and along the axis of the cast part. The section of a casting where the grains are competing to establish parallel growth is unusable as a turbine part and must be discarded.
A need exists for a directional solidification process that can produce columnar castings characterized by prescribed grain spacings. Additionally, a need exists for a directional solidification process that provides an increased proportion of casting characterized by aligned parallel axis oriented crystals.
The invention relates to a method of controlling grain spacing of a casting and to the product of the method. In the method, a grain starter that is capable of nucleating a multiplicity of grains, is positioned within a mold. The mold is filled with molten metal and a solidification interface is caused to pass from the grain starter through the molten metal by immersing the mold in a cooling bath to form a casting that has a multiplicity of grains nucleated by the grain starter.
In an embodiment, a grain spacing is determined for a columnar article. A grain starter is selected that has a grain spacing determined to provide the grain spacing in the article when the article is cast in a liquid metal cooled directional solidification process. The grain starter is positioned in a mold and the mold is filled with molten metal. A solidification interface is caused to pass from the grain starter through the molten metal by immersing the mold in a liquid metal cooling bath to form an article having a grain spacing determined by the grain starter spacing.
In still another embodiment, the invention relates to a method of producing a cast article with a substantially increased proportion of parallel axis grain structure. The method comprises making a fine grain superalloy multicrystalline grain starter capable of nucleating a multiplicity of grains in a solidifying casting. The grain starter is provided within a mold and the mold is filled with molten metal. A solidification interface is then caused to pass from the grain starter through the solidifying casting by immersing the mold in a cooling bath to form a casting that has a multiplicity of grains nucleated by the grain starter.